PTC panel heater with small rush current characteristic and highly heat insulating region corresponding to heater location to prevent local overheating

ABSTRACT

A panel heater comprising, on an upper surface of a metallic sheet (2) provided with PTC ceramics (1) on the back side thereof, a highly insulating material or an open space (4) at the portion corresponding to the back side portion to which the PTC ceramics are provided, with a material (3) for conducting heat from the metallic sheet being thus provided to the rest of the surface side of said metallic sheet, and a finishing sheet (5) being established on the upper surface of the whole structure. More preferably, the metallic sheet (2) is provided with equally spaced concave grooves with PTC ceramics (1) on the back plane of said grooves and a highly insulating material or an open space (4) inside said grooves, and a finishing sheet (5) established thereon. Accordingly, a simply structured panel heater resistant to heavy loads yet free from local overheating is provided.

FIELD OF ART

The present invention relates to a PTC (positive temperaturecoefficient) thermistor device using a PTC thermistor element; moreparticularly, it relates to a panel heater.

BACKGROUND OF THE INVENTION

Panel heaters are now applied to various uses. In particular, panelheaters using PTC thermistors as heating elements are free fromoverheating, and hence enjoy advantages such as automatic self controlof calorific value even under change of surrounding temperature.Accordingly, they are used as components for general use heaters, suchas food warmers for hospitals, and for numerous other purposes.Technology of this type is disclosed as panel heaters in JapaneseUnexamined Patent Publication No. 61-256123 and Japanese UnexaminedPatent Publication No. 62-16994.

The panel heaters as disclosed in the aforementioned unexaminedpublished Japanese patent applications, however, suffer problems. Thoseproblems include the fluctuation of temperature due to local heat justabove the heater; insufficient heat conduction from the heater elementto the panel due to the presence of thermal resistance at the joint; thenecessity of additional process steps, such as bolting, for the joints;and the need for a complicated structure of reinforcing materials whichare incorporated into the heater to support the heavy load being appliedfrom the upper side of the heating element.

Furthermore, on installing a panel heater comprising a combination of aPTC thermistor and a metallic heat radiation sheet to materials forwalls and floors, a PTC thermistor sintered element capable of coveringa large area is a requisite. In practice, however, large area panels areunfeasible, because such a large sintered element for a PTC thermistorwill require great difficulties in manufacturing. Even if an alternativeprocess for realizing a large area panel heater were to be taken, i.e.,integrating smaller PTC thermistor sintered elements into a larger one,the process would be uneconomical and energy-insufficient. Moreover,such a sintered element obtained by integrating smaller pieces of PTCthermistor sintered elements is not practical, because a large heatemission occurs preferentially at the joint portions of the PTCthermistors with a metallic sheet.

An advantage of a PTC thermistor is a quick temperature rise which isrealized as a consequence of the rush current (a large initial surge ofcurrent which is generated immediately after applying the current).However, the rush current from a plurality of PTC thermistors willaccumulate into a large current that has unwanted effects such asactivation of the breaker.

An object of the present invention is to overcome the prior art problemsas mentioned hereinbefore, and to provide a simply structured panelheater resistant to heavy loads, and free from local overheating.

Because panel heaters are characterized by their thin sheet-likestructure, they have been utilized as general use heaters, floorheating, etc. Accordingly, the PTC thermistor heaters tend to be usedmore frequently because they have a self control function fortemperature, which provides enhanced safety.

Conventional PTC thermistor heaters for use in floor heating were planarheaters comprising a heat-resistant insulating organic material havingconductive materials such as carbon particles dispersed therein.

Those conventional planar heaters, however, suffered the followingdrawbacks: (1) Lack of stability in heat emission--the contact state ofthe conductive material particles changes along with the changingtemperature, providing no assurance that the initial contact state willrecover upon return to the initial temperature; (2) Non-uniform heatertemperature--as mentioned above, because the contact state of theconductive particles is non-uniform, the electric resistance differsfrom one place to another; and (3) Difficulty in placing the panelheater--because virtually the entire front panel is covered with aheating element, the positions for safely nailing up the panel arehighly restricted.

Accordingly, a second object of the present invention is to provide apanel heater which stably emits heat without suffering non-uniformtemperatures, and which can be freely fixed to a floor using a nail orthe like. Furthermore, it is also an object of the present invention toshorten the transient time from when the electric source is turned on tothe point the rush current reaches the panel heater.

A PTC thermistor has a low initial resistance, and it is known that alarge rush current generates on it when it is used as a heating element.Accordingly, a larger current capacity is required for the initial stagethan that required for the stationary state. This signifies that, when aPTC thermistor is applied to panel heaters or other uses which require alarge output, the output and the number of panel heaters must beconfined to a certain range.

To cope with the above problems, much effort has been put into reducingthe rush current to as low a value as possible.

Such efforts are described in Japanese Unexamined Patent Publication No.55-97143, in which a PTC thermistor being serial connected with anegative temperature coefficient thermistor is disclosed, or in JapaneseUnexamined Patent Publication No. 54-115443, in which an ohmic contactbeing connected with a non-ohmic contact is disclosed. Furthermore,Japanese Unexamined Patent Publication No. 49-27932 discloses a combineduse of PTC thermistors differing in Curie point, and Japanese UnexaminedPatent Publication No. 63-218184 discloses the use of a phasetemperature control device.

The conventional techniques as cited above are disadvantageous in thatthey incorporate additional process steps that make the circuit morecomplicated. In particular, panel heaters and the like as disclosed inJapanese Unexamined Patent Publication No. 49-27932 suffer fromconsiderable temperature fluctuation.

A third object of the present invention is, therefore, to control therush current by the thermistor itself without using any additionalcircuits and elements.

Heating elements obtained by joining and electrically connecting aplurality of PTC thermistors with two electrodes are used practically insuch items as fan heaters, hair driers, and bedding driers. The outputpower of a device using such a heating element is controlled by theplacement of a plurality of those heating elements each composed of PTCthermistors joined and connected with two electrodes, and thus applyingthe current to only the selected heating elements. The output power isthus controlled by the number of heating elements to which the currentis applied.

However, to increase the output in heating elements of the above type, alarger number of heating elements must be incorporated. The installationof these additional elements will require not only more space, buthigher cost as well.

A fourth object of the present invention is, therefore, to provide a PTCthermistor heater composed of a plurality of PTC thermistors which areintegrated into a single heating element, capable of changing the outputby itself.

A fifth object of the present invention is to avoid loss of strength ofthe PTC thermistor heater due to the spaces which results from the PTCthermistor elements being arranged leaving spaces in the heatingelement, and also to prevent fire or breakage from occurring due to ashort circuit.

SUMMARY OF THE INVENTION

The first object of the present invention can be achieved by realizing apanel heater as follows. That is, the present invention provides a panelheater comprising: a metallic sheet provided with PTC ceramics on theback plane thereof; a high heat insulating material or an open space onthe surface side of said metallic sheet, at the portion corresponding tothe portion on the back side which has the PTC ceramics; materials forconducting heat from the metallic sheet, each arranged to predeterminedpositions on the surface side other than those positions at which theheat insulating material or the open space is provided; and a finishingsheet being provided on the surface side of said materials forconducting heat from the metallic sheet. The present invention alsoprovides a panel heater comprising a metallic sheet having providedthereon concave grooves at predetermined spaces by bending, with PTCceramics being placed on the back side of said concave grooves and aheat-insulating material or a space being placed inside said concavegrooves, and a finishing sheet being placed on the outermost surface ofthe structure.

In another embodiment, a plurality of PTC ceramics may be dividedarbitrarily into groups, and a thermostat placed in each of said groups.In such a structure, the thermostat provided to the first group of PTCceramics closes first on application of a current for elevating thetemperature of the PTC ceramics, and hence the current thereafter isapplied to the PTC ceramics in the next group. With such a structure,the current may be applied sequentially to the PTC ceramics in the nextgroup.

The metallic sheet is heated by applying current to the plurality of PTCceramics which are adhered to the back side of the metallic sheet. Theheat propagates through the metallic sheet by conduction, but astationary state of non-uniform temperature is realized because thejoint portion of the metallic sheet with the PTC ceramics is maintainedat the highest temperature while the portion furthest from the jointremains at a low temperature. Thus, the panel heater according to thepresent invention comprises a material for conducting heat at lowtemperature portions furthest from the joint portion of the metallicsheet with the PTC ceramics, and a heat-insulating material or an openspace at high temperature portions which correspond to the vicinity ofthe joint portion of the metallic sheet with the PTC ceramics. In thismanner, the finishing sheet provided to the surface of the wholestructure receives more heat at portions furthest from the joint portionof the metallic sheet with the PTC ceramics by allowing the heat to bemore conducted to these portions, while it is less heated at the portionof the finishing sheet just above the joint portion of the metallicsheet with the PTC ceramics by allowing the heat there to be lessconducted. Thus, the uneven temperature of the finishing sheet can bemade more uniform. In an embodiment in which an open space is providedto the upper side portion corresponding to the joint portion of the PTCceramics with the metallic sheet, the heat generated from the joint canbe conducted to remote portions by the convection of air if a spaceconnecting the remote portions with the aforementioned open space isprovided. In this manner, the unevenness of the temperature can befurther reduced.

To prevent an excess rush current from occurring when heating the entirePTC ceramics, the PTC ceramic elements are divided into a plurality ofgroups each having a thermostat provided thereto. Then, current can beapplied sequentially to the groups of PTC ceramics using the thermostat.

The second object of the present invention can be achieved by a thinsheet panel heater comprising PTC ceramic elements. The thin sheet panelheater according to the second embodiment of the present inventioncomprises a panel having a concave portion provided on the upper surfacethereof to accommodate therein the heating elements, connection wires,etc.; a plurality of heating elements comprising ceramic heaters andcomponents such as connection wires attached thereto; and a heatradiation sheet to cover the entire surface of the panel.

In the structure above, the plurality of PTC ceramic elements beingincorporated between the upper and the lower electrode sheets may bedivided into a number of groups so that the PTC elements within a groupmay be brought into intimate thermal contact with each other.

The heating element for use in the panel heater according to the presentinvention comprises heaters made of PTC ceramics, (for example, bariumtitanate (BaTiO₃)). Barium titanate is a semiconductor ceramic having apositive temperature coefficient of resistance. The electricalresistance of the barium titanate sintering increases non-linearly withan elevating temperature by applying a current thereto, as to reach avalue 10,000 times that of the room temperature at a temperature notlower than the Curie point. Over this temperature, the ceramics willturn into an insulator that will not conduct current. Consequently, thetemperature of the sintering element is maintained constant at atemperature slightly higher than the Curie point. It is well known thatthe Curie point can be freely controlled by adding elements such as leadand strontium to the ceramic sintering element.

The PTC ceramic element thus obtained is characterized in that it hasself controlling functions for both temperature and power output. Theformer function assures safety, while the latter largely contributes tosaving energy. Accordingly, as these elements have been long used asheating elements for fan heaters, hair driers, driers for bedding, etc.,the stability and durability of the PTC ceramic elements are now wellestablished.

It can be therefore be said that the PTC ceramic elements are bestsuited for use in panel heaters for home-use floor heating.

For easier fabrication and temperature control, the aforementionedheating element is preferably constructed from a plurality of small PTCceramic element blocks. In general, such PTC ceramic elements arearranged at even spacing. However, the resulting heating element willtake longer to attain the Curie point because the heat generated on eachof the PTC ceramic elements is consumed for heating the electrodes whichcover the large area. Thus, it happens that, the rush current, i.e., thetransient current which is larger than that which is generated in astationary state after the power source has been turned on, remainslonger for a heating device equipped with a larger number of PTCelements. As a means to overcome this problem, the PTC ceramic elementsare arranged, not by equal spacing, but by combining them into aplurality of groups and bringing the elements closer to each otherwithin each of the groups. In this manner, the temperature of each ofthe elements can be more rapidly increased, and the transient statecorresponding to the rush current, i.e., the period between when thepower source is turned on and when the stationary temperature isachieved, can be minimized.

The third object of the present invention can be achieved by a panelheater equipped with a PTC thermistor which satisfies the followingrelation on applying an AC voltage of 100±5 V: P×R≧2.0×10⁴ (W·Ω),wherein P(W) represents the output power in a stationary state at roomtemperature, i.e., at 25° C., and R(Ω) represents the resistance beforeapplying the voltage (nominal resistance).

In the panel heater according to the present invention, the product ofthe resistance and the output of the PTC thermistor is restricted to acertain value or higher to considerably reduce the rush current. Thereason for defining the relation P×R≧2.0×10⁴ (W·Ω) is explained below.

FIG. 11 shows the change in current of PTC heaters with the passage oftime. The PTC heaters used herein are those having a nominal resistanceof 50, 70, 110, 165, 200, and 260 Ω. In the figure, the characteristiccurve marked with c is for a heater having a resistance of 110 Ω. It canbe seen that the rush current in this heater amounts to 1.25 times thecurrent at the stationary state, and that it reaches a peak after thepassage of 3 minutes. Considering the functioning characteristics of ahome-use breaker, which operates only after the passage of 4 minutesunder a current amounting to 125% of the rated current, the heater abovecan be operated without activating the breaker. Accordingly, thecharacteristic curve marked with c was referred to as criticalcharacteristics.

The fourth object of the present invention can be achieved by a PTCheater comprising a plurality of PTC thermistors each having one of thepoles electrically connected with a common principal electrode, and theother pole selectively with electrodes for applying a current so thatthe functioning thermistors can be selected, provided that the electricconnections with the plurality of electrodes are not superposed in thesame plane for a single thermistor.

The fourth object of the present invention can be achieved by anotherPTC heater comprising a plurality of PTC thermistors each having each ofthe poles electrically connected with electrodes to select thefunctioning thermistors, provided that the electric connections with theplurality of electrodes are not superposed in the same plane for asingle thermistor.

In general, the PTC heaters are used by attaching the PTC thermistors toa pertinent heat-radiant member, incorporating therebetween anelectrode. It is known that the radiant heat, i.e., the power output,differs depending on the area of the PTC thermistor to which theelectric current is applied for elevating the temperature, and that theradiant heat or the power output is higher for a larger area of thethermistor. Thus, when a plurality of PTC thermistors are integratedinto a single heating element, the radiant heat and the power output ofthe heating element can be varied by changing the number of PTCthermistors to which the current is applied. This can be achieved by astructure comprising a plurality of flat PTC thermistors, eachthermistor being connected to one or more electrodes for applying acurrent to one of the two principal planes thereof and two or moreelectrodes to the other plane, provided that the plurality of theelectrodes for applying the current is arranged in such a manner thatthe electric junctions are not superposed on each other for a singlethermistor.

In the structure above, a desired PTC thermistor can only be heated byapplying a current to the two electrodes, each one being selected fromthe electrodes connected to each of the two principal planes.

By thus selecting one electrode each from those provided to each of thetwo principal planes and combining them, the number of the exothermicPTC thermistors can be changed to control the radiant heat and the poweroutput of the entire heater.

The fifth object of the present invention can be achieved by a PTCheater device comprising a plurality of PTC elements being arranged witha predetermined spacing between one another and being incorporatedbetween parallel upper and lower band-shaped electrode sheets, whereinan insulator material is provided to the open space corresponding tosaid spacing.

In the present invention, an insulating sheet having a thicknessapproximating that of the PTC element was placed in the space betweenthe upper and the lower parallel electrode sheets. Because the electrodesheets are supported by the insulating sheet in this manner, theelectrode sheet is free from bending even when a load is applied to thePTC elements.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a planar view, with parts broken away, of a panel heateraccording to Example 1 of the present invention;

FIG. 2 is a cross-sectional view of the panel heater taken along lineII--II shown in FIG. 1;

FIG. 3 is an explanatory figure of a structure having a PTC heater shownin FIG. 1 being attached thereto;

FIG. 4 shows the connections between the PTC heater shown in FIG. 1 witha thermostat;

FIG. 5 is an explanatory cross sectional view of a panel heateraccording to Example 2 of the present invention;

FIG. 6 is a front view of a panel heater according to Example 3 of thepresent invention;

FIG. 7 is a cross sectional view of the panel heater taken along lineVII--VII shown in FIG. 6;

FIG. 8 is an exploded perspective view of a panel heater according toExample 4 of the present invention;

FIGS. 9a, 9b and 9c are perspective views of various types ofband-shaped heating elements;

FIG. 10 is a graph showing the change of current with the passage oftime for each of the heating elements differing in structure;

FIG. 11 is a graph of characteristic curves showing the change incurrent with the passage of time for PTC heater devices according toExample 5, differing in nominal resistance;

FIG. 12 is a graph showing the influence of α value on theresistivity-temperature characteristics;

FIG. 13 is a graph showing the influence of α value on the time-currentcharacteristics at a resistance of 110 Ω;

FIG. 14 is a graph showing the influence of α value on the time-currentcharacteristics at a resistance of 260 Ω;

FIG. 15 is a graph which relates the α value to withstand voltage;

FIG. 16 is a graph showing time-current characteristics with a changingresistivity ratio;

FIG. 17 is a graph showing the influence of the addition of Nb₂ O₅ onthe time-current characteristics;

FIG. 18 is a planar view of a PTC heater according to Example 6 of thepresent invention;

FIG. 19 is a left side view of a PTC heater according to Example 6 ofthe present invention;

FIG. 20 is a right side view of a PTC heater according to Example 6 ofthe present invention;

FIG. 21 is a cross sectional view of a PTC heater according to Example 6of the present invention, taken along line 21--21 of FIG. 18;

FIG. 22 is a cross-sectional view of a PTC heater according to Example 6of the present invention, taken along line 22--22 of FIG. 18; and

FIG. 23 is a perspective view of a PTC thermistor heater device of thepresent invention according to Example 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be explained in further detail referring tothe examples and attached figures.

EXAMPLE 1

FIG. 1 is a planar view with parts broken away of a structure comprisinga metallic sheet 2 having joined thereto a PTC heater element 1 with aniron finishing sheet 5 being provided thereon, and an aluminum heatconductor sheet 3 and an open space 4 being incorporated therebetween.The finishing sheet may be a material other than a metal.

A thermostat 14 is provided at the vicinity of the PTC element 1 tocontrol the current which is applied to the neighboring PTC heatingelement. In this manner, electric current is applied to the next PTCelement which is provided adjacent to the PTC heating element 1 onlyafter the temperature of the PTC heating element 1 exceeds a previouslyset value.

The flooring material 6 may be connected as desired using connection 13,so that, as a whole, a parallel connection is provided.

FIG. 2 details the cross-sectional structure of the PTC panel heater, astaken along line II--II of FIG. 1. As is shown in FIG. 2, the metallicsheet 2 having the PTC heating element 1 adhered thereon is fixed on aheat insulating material 7, and an open space 4 is provided at thevicinity of the joint portion of the metallic sheet 2 with the PTCheating element 1, while an aluminum heat conductive plate 3 is fixed tothe portion remote from the joint portion with the PTC heatingelement 1. An iron finishing sheet 5 is provided to cover the wholestructure.

The heat conductive plate 3 may be composed of a plurality of metallicsheets, but not necessarily of a heat conductive material such as ametal. It may also comprise features such as holes and grooves which cancommunicate with space 4.

FIG. 3 shows the detailed structure of the PTC heating element 1 as itis adhered to the metallic sheet. In general, the PTC heating element 1comprises a plurality of PTC thermistor sintered elements 8. In the samefigure, there can be seen aluminum electrodes 9 and 10 being attached tothe upper and the lower surfaces of the PTC thermistor sintered elements8. Those electrodes emit heat on the application of a current theretousing a connection 13. A ceramic insulator board 11 is attached to theupper aluminum electrode using a heat-resistant adhesive 12. Theresulting structure is placed under the metallic sheet 2.

FIG. 4 shows the connection between the PTC heating element and thethermostat for controlling the current. A thermostat 14 is provided atthe vicinity of a PTC heating element 1 that, upon heating the PTCheating element 1, by applying thereto a current, to a predeterminedtemperature or higher, the thermostat 14 closes to apply a current tothe adjacent PTC heating element to initiate the heating thereof.

In the arrangement shown in FIG. 4, the PTC heating elements are heatedsequentially from the left side. By lowering the rush current of the PTCheating element by controlling the resistance and such thereof, thethermostat 14 need not be connected to each of all the PTC heatingelements, but simply installed in the vicinity of a particular pluralityof PTC heating elements. In practice, panel heaters according to thepresent invention are set and fixed to the floor in the desired numbers,connected electrically, and covered with a finishing material, a carpet,etc.

EXAMPLE 2

FIG. 5 is a cross sectional view taken at the edge of a panel heateraccording to Example 2, which comprises a metallic sheet provided withstripe grooves. In this type of a panel heater, a PTC heating element 1is adhered to the back of the stripe grooves, and an open space 4 isprovided inside the grooves. A finishing sheet 5 covers the stripegrooves incorporating therebetween the open space 4, and the metallicsheet at portions other than the stripe grooves.

EXAMPLE 3

FIG. 6 is a front view of an embodiment according to Example 3 of thepresent invention, which comprises a panel heater 15 being fixed to asupport 16. FIG. 7 is a cross-sectional view of the structure takenalong line VII--VII of FIG. 6. Three heating elements 1, each comprisinga plurality of PTC thermistors, are sandwiched between two aluminuminternal heat radiation sheets 2 and joined. The internal heat radiationsheets 2 are joined with aluminum external heat radiation sheets 5, withaluminum heat conductive sheets 3 being incorporated therebetween. Anopen space 4 is provided on each of the portions of the internal heatradiation sheets to which the heating element 1 is attached, so that theheat from the heating element 1 will not be conducted directly to theexternal heat radiation sheet 5. Thus, the open space 4 not onlyfunctions as a funnel to provide a higher heat radiation, but alsoprovides a clean and safe panel heater which is free from accidents suchas burning. This can be achieved by selecting a material having anappropriate Curie point for the PTC thermistor, and thereby preventinglocal overheat from occurring on the external heat radiation sheet 5.

As described in the foregoing, the present invention provides a panelheater equipped with a PTC thermistor as the heating element. Becausethe PTC thermistor itself has an automatic self temperature controlfunction, the resulting panel heater controls the power consumption inaccordance with the outer temperature. Thus, the panel heater is notonly safe, but also energy efficient. The metallic sheet is monolithic,and hence it provides high heat conductivity to prevent the heat frombeing conducted directly to the finishing sheet from the heatingelement. Accordingly, the metallic sheet suffers no local overheat, and,although it has a large area and a simple layered structure, it resistsa large load without incorporating any reinforcements or the like.

Furthermore, an excess rush current can be avoided by using a structurecomprising the heating elements being divided into a plurality of groupseach provided with a thermostat. By using such a structure, each groupof heating elements can be heated sequentially by applying thesequential current.

EXAMPLE 4

FIG. 8 is an exploded perspective view of the panel heater according toExample 4 of the present invention, which comprises a panel 21. In thepresent Example, the panel 21 comprises a lower base panel 22 made ofplywood, etc., which is laminated with another panel 23 made of the samematerial. To the upper panel 23 are provided a plurality of notches (twonotches in the present example) 23a to accommodate therein band-shapedheating elements and accompanying components such as connections. Then,a band-shaped heating element 24, a connection 25, and a thermal fuse 26are each placed inside the notches 23c, respectively, and connected. Thewhole structure thus obtained is then covered with a heat radiationsheet 27 such as one made of aluminum, and finished into a panel heaterby adhering them together. A plug 28 to supply electric power to thepanel heater from an electrical outlet is also provided along edge 23b.

FIGS. 9a, 9b and 9c are perspective views of various types ofband-shaped heating elements. The structure shown in FIG. 9(a) comprisesa plurality of PTC ceramic elements 31 equally spaced out at a spacing34 and being arranged between upper and lower electrode sheets 32 and33. The PTC ceramic elements 31 are adhered to the upper and the lowerelectrode sheets 32 and 33. The structure shown in FIG. 9(b) comprises aplurality of PTC ceramic elements 31 being divided into three or moregroups, with the elements in each group being brought into close contactwith the others within the group. FIG. 9(c) illustrates an extreme casewhich comprises two groups of PTC ceramic elements, where the groups arewidely separated from each other; i.e., one at each end of thestructure.

FIG. 10 shows the change of current upon application of rated voltage tothe panel heater comprising the PTC ceramic elements 31 arranged asdescribed above. As can be seen from the figure, the structurecomprising the equally spaced PTC ceramic elements as shown in FIG. 9(a)takes 300 seconds or longer to achieve a current of 2 A from the initialcurrent in the range between 3 and 4 A. In the case of a structure asshown in FIG. 9(b), which comprises three heating elements composed ofPTC ceramic elements, the corresponding transient time is reduced toabout 60 seconds, and in the structure having two heating elements asshown in FIG. 9(c), the transient time is further reduced to a mere 40seconds. This is because the heat generated by the PTC ceramics is lessdiffused in the structures as shown in FIGS. 9(b) and 9(c). Accordingly,the temperature rises more rapidly in the PTC elements arranged in suchstructures, and the elements thereby attain the Curie point faster andhave a shorter rush current period.

It should be noted, however, that in the arrangement shown in FIG. 9(c),it becomes difficult to achieve a uniform temperature over the panel. Itis therefore necessary to determine the number of elements per group toobtain an optimum rush current period and a uniform temperature over thewhole panel.

As mentioned in the foregoing, the Example according to the presentinvention provides the following advantages:

(1) A safe and durable panel heater is provided, because PTC ceramicelements, which are more stable in heat emission, are used as theheating elements;

(2) A low cost panel heater is provided by a simpler process, becauseplywood is used as the base panel;

(3) A panel resistant to heavy loads is provided, because the heatingelements are buried inside the plywood base panel, and thereby the loadof the flooring is applied to the plywood;

(4) A panel with excellent electrical and heat insulation is obtained,because the heating elements are buried inside the plywood;

(5) A panel heater which can be easily attached to the floor isprovided, because the panel may be nailed at almost any place asdesired; and

(6) A panel heater having a considerably reduced rush current period isobtained by placing the plurality of PTC ceramic elements in aparticular group arrangement; accordingly, a number of panel heaters canbe used at once and yet with a quick temperature rise, withoutactivating the breaker.

EXAMPLE 5

The present Example provides a panel heater having a considerablyreduced rush current of about 30 to 70% of that of a conventional one.This was achieved by modifying the characteristics of the PTC elementsand increasing the resistance per unit output of the panel heater.

In a panel heater 900×900×13 mm in size, for instance, it has been foundfor those yielding curves c to f as shown in FIG. 11, that the rushcurrent can be greatly reduced by changing the resistance withoutconsiderably influencing the power output. It has been also found thatthe resistance is affected by varying the α value, i.e., an index fordefining characteristics of a PTC element.

In the present Example, the temperature coefficient of resistance α isdefined as follows:

α=[2.303 log(R₂₀₀ /R₂)/{(T₂₀₀ -T₂)×10}] where, T₂₀₀ and T₂ represent thetemperature at which the resistance is increased respectively to 200times and 2 times that at 25° C.; and R200 and R₂ represent theresistance respectively 200 times and 2 times that at 25° C.

The resistance-temperature characteristic curves at a constantresistance and with varying α value are given in FIG. 12. Thetime-temperature characteristic curves at a constant resistance and withvarying α value are given in FIGS. 13 and 14. As those figures read, therush current increases with a larger α value to yield a curve having asharper maximum.

The phenomena above can be explained as follows. The PTC elements havinga higher α value have a rapid temperature rise and a distinct currentlimiting effect. However, those elements are more susceptible to heataccumulation at the central portion of the element, i.e., the portionwhich is heated most rapidly, and thereby the rush current increases dueto the pinching effect. It can be seen therefore that the α value bepreferably lowered to as low a value as possible. However, as seen inFIG. 15, the withstand voltage reduces with a decreasing α value. It istherefore regulated to keep the distance between the electrodes to 2.5mm or longer. Furthermore, if a voltage of 100 V were to be applied, awithstand voltage of about twice that is necessary. This signifies thatthe minimum allowable α value is 5%/°C. If a predetermined constantcurrent were to be obtained at an α value being increased to 20%/°C. orhigher, the rush current increases excessively to activate breaker.Thus, it would be undesirable to increase the α value to 20%/°C. orhigher.

The rush current may be reduced by allowing the heat generated on thePTC element to diffuse rapidly. In this case, the heat diffusion isdelayed if the heat generation occurs mainly from the central portion ofthe element, because the heat conductivity of the PTC element is not sohigh as that of a metal. This requires that the central portion of theelement does not have a high resistivity as measured along the directionof thickness. The time-current characteristic curves with varyingresistivity are shown in FIG. 16.

As FIG. 16 shows, the rush current increases even when the resistance ismaintained at a constant, with an increasing ratio of the resistivity ofthe surface portion to the resistivity of the inner portion. It can beseen further that the curve at a ratio of 1.1 coincides with the breakercharacteristics, and that therefore the ratio should be maintained nohigher than 1.1.

The term "resistivity of the surface portion" as mentioned abovesignifies the resistivity of the portions falling within one third ofthe distance between the electrodes as measured from the two surfaceelectrode sides, and the term "resistivity of the inner portion"signifies that of the central portion falling in the remaining one thirdof the distance between the electrodes.

The foregoing description referred to a PTC element having a Curie pointof 110° C., but similar descriptions can be given to those having Curiepoints of 40° C. and 70° C., allowing for an increase in the surfacearea and the number of PTC elements to compensate for the drop in outputpower.

The PTC ceramic elements are fabricated from high purity materials, andthe properties thereof are controlled by doping trace amounts ofimpurities such as SiO₂ and MnO₂. Among the impurities, PbTiO₃ andSrTiO₃ are temperature shifters which are added for the purpose ofshifting the Curie point of the PTC ceramics; SiO₂, TiO₂, etc., areadded for controlling the grain size to increase the withstand voltage;and the compounds of transition metal elements such as MnO₂ are intendedto increase the α value and to increase the withstand voltage.

Though high purity starting materials are used, components which areextremely difficult to separate and those having no substantialinfluence on the characteristics of the resulting sintering were leftintact.

Niobium is one such inseparable element which is mostly included inTiO₂, one of the raw materials for PTC ceramics, in the form of an oxideNb₂ O₅ or Nb₂ O₅₋δ. In general, the elements for rendering the sinteringsemiconductive are added at an amount of 0.1 to 0.3% by weight. BecauseNb₂ O₅ functions the same as such elements, Nb₂ O₅ or other rare earthelements are added while taking the amount of the inherent Nb contentinto account.

It has been found, however, that Nb₂ O₅ has an unfavorable influence onreducing the rush current. FIG. 17 is a graph showing the time-currentcharacteristics of a PTC element with changing Nb₂ O₅ addition in fourlevels, i.e., 0.0% by weight, 0.015% by weight, 0.030% by weight, and0.045% by weight. As FIG. 17 reads, the rush current increases with anincrease of Nb₂ O₅ in obtaining the same stationary current. The effectof the Nb₂ O₅ addition is yet to be clarified. However, it is believedthat Nb₂ O₅ cannot be incorporated into the ceramic sintering in theform of a completely uniform solid solution because of its highactivity, and that it is apt to remain in the grain boundaries upon thecooling of the sintering.

Ideally, a single barrier layer is formed in the ceramic sintering.However, a layer comprising Nb₂ O₅ as the principal component is formedat the same time, and it behaves as an ordinary semiconductor layer.That is, the PTC ceramics as a whole become more dependent on voltageand allow more current to be conducted than the current defined by theapparent resistance-temperature characteristics. In this manner, therush current increases along with the additions of Nb₂ O₅.

The change of current with passage of time was measured on a panelheater having a structure as shown in FIG. 8, while varying theresistance (before applying voltage) of the PTC element to yield a panelresistance of 50, 70, 110, 165, 200, and 260 Ω. The results are given inFIG. 11.

As shown in FIG. 11, the panel heaters yielding curves a and b have toolarge a rush current with respect to the stationary current, to thepoint that a current below the activating point of the breaker cannot beachieved within 4 minutes. Thus, panel heaters yielding curves c andbelow were selected for use.

The product P×R for the curves a to f can be calculated from thecharacteristics which can be read on FIG. 11 as follows.

Curve a (having a resistance of 50 Ω): P×R=1.06×10⁴

Curve b (having a resistance of 70 Ω): P×R=1.35×10⁴

Curve c (having a resistance of 110 Ω): P×R=2.00×10⁴

Curve d (having a resistance of 165 Ω): P×R=2.84×10⁴

Curve e (having a resistance of 200 Ω): P×R=2.98×10⁴

Curve f (having a resistance of 260 Ω): P×R=3.59×10⁴

If a home-use breaker with a maximum allowable current range of 12 Awere to be serially connected, it can be shown by simple calculationthat five panels, each represented by the characteristic curve a andeach having a current demand of 2.12 A, can be installed. However,because this panel has a rush current as large as 3.9 A as read fromFIG. 11, the breaker will activate if five panels are connected. Inpractice, the maximum allowable number of such panels has been three.Similarly, up to three panels, each having a characteristic curve b, aregenerally allowed to be connected.

In contrast to the cases above, the panels according to the presentExample and represented by the characteristic curves c to f yieldednegligible rush current, or a rush current well below that which wouldactivate the breaker. Accordingly, those panels can be specified by theoutput of the stationary state.

More specifically, the panels defined by the characteristic curves c tof were subjected to experimentation to see how many panels could beconnected to a home-use breaker. It has been found as a result that sixpanels for those of curves c and d, and eight panels for those of curvese and f, can be connected. This is a 2 to 3-fold improvement over thosepanels exhibiting characteristics with curves a and b.

In the present Example, PTC elements of 2.5 mm thickness were used.Thicker PTC elements may be used without any restriction, but those of1.5 mm or less are unfavorable because they may suffer low withstandvoltage or an increased rush current due to the ease in conducting heatfrom the central portion of the element to the heat radiation side.

As explained in the foregoing, the present invention provides a panelheater of considerably reduced rush current. Accordingly, with such apanel heater, the number of panels allowable for connection may be setin accordance with the stationary output power of the panel heater. Suchpanel heaters, as a consequence, can heat a much larger area.

EXAMPLE 6

FIG. 18 is a planar view of a PTC heater according to Example 6 of thepresent invention, and FIGS. 19 and 20 are each a left side view and aright side view of the same PTC heater, respectively. As shown in FIG.18, the PTC thermistors 41 to 46 are connected by the upper side thereofwith the upper left electrode sheet 47 and upper right electrode sheet48. The lower sides of the PTC thermistors 41 to 46 are connected to thelower left electrode sheet 49 and the lower right electrode sheet 50.The electrode sheets 47, 48, 49, and 50 are each electricallydisconnected with each other.

FIG. 21 is a cross sectional view of the PTC heater above, taken alongline 21--21 of FIG. 18. The upper side of the PTC thermistor 41 isadhered to the upper left electrode sheet 47 using an electricallyconductive adhesive 51 and an insulating adhesive 52 to assure theelectric connection. The lower side of the PTC thermistor 41 isconnected electrically to the lower left electrode sheet 49 in the samemanner as the upper side of the same thermistor. Both of the principalplanes of the PTC thermistor 41 are covered with an electrode film 53.

FIG. 22 is a cross sectional view of the same PTC heater, taken alongline 22--22 of FIG. 18. The upper side of the PTC thermistor 41 iselectrically insulated, but connected to the upper electrode sheet 48using an insulating adhesive 52. Similarly, the lower side of the PTCthermistor 41 is electrically insulated and adhered to the lower rightelectrode sheet 50.

The electric connection and disconnection of each of the PTC thermistorswhich can be realized by selecting the connecting electrode sheets areshown in Tables 1 and 2 below. Table 1 shows the result for the cases inwhich a single electrode sheet is selected, and Table 2 shows those inwhich a combination of two or more electrode sheets are selected. Bycombining the selected electrodes for applying the current, the numberof the PTC thermistors to be turned on can be selected to change theheat radiation and the output power of the PTC heaters.

                  TABLE 1                                                         ______________________________________                                                 Connection state                                                                          Combination of Electrodes                                         of Electrodes                                                                             to which Current is applied                              Electrode No.                                                                            7     8      9   10   7-9  7-10 8-9  8-10                          ______________________________________                                        PTC thermistor 1                                                                         ◯                                                                       ×                                                                              ◯                                                                     ×                                                                            ⊚                             PTC thermistor 1                                                                         ×                                                                             ◯                                                                        ×                                                                           ◯       ⊚              PTC thermistor 1                                                                         ◯                                                                       ×                                                                              ×                                                                           ◯                                                                           ⊚                        PTC thermistor 1                                                                         ×                                                                             ◯                                                                        ◯                                                                     ×        ⊚                   PTC thermistor 1                                                                         ◯                                                                       ×                                                                              ×                                                                           ◯                                                                           ⊚                        PTC thermistor 1                                                                         ×                                                                             ◯                                                                        ×                                                                           ◯       ⊚              Number of heated PTC thermistors                                                                   1      2      1    2                                     ______________________________________                                         Note:                                                                         ◯: Electrically connected;                                        ×: Insulated;                                                           ⊚: PTC thermistor to be heated by applying current.       

    __________________________________________________________________________                Connection state of Electrodes                                    Electrode No.                                                                             (7 + 8) - 9                                                                         (7 + 8) - 10                                                                        (7 + 8) - 9                                                                         (7 + 8) - 9                                                                         (7 + 8) - (9 + 10)                        __________________________________________________________________________    PTC thermistor 1                                                                          ⊚                                                                          ⊚                                                                          ⊚                          PTC thermistor 2  ⊚                                                                          ⊚                                                                    ⊚                          PTC thermistor 3  ⊚                                                                    ⊚                                                                          ⊚                          PTC thermistor 4                                                                          ⊚  ⊚                                                                    ⊚                          PTC thermistor 5  ⊚                                                                    ⊚                                                                          ⊚                          PTC thermistor 6  ⊚                                                                          ⊚                                                                    ⊚                          Number of Thermistors                                                                     2     4     3     3     6                                         __________________________________________________________________________     Note:                                                                         ⊚: PTC thermistors to be heated by applying current       

In the Example above, two electrodes were provided on each of the twoprincipal planes. However, a common electrode may be provided to one ofthe principal planes of all the PTC thermistors, and a plurality ofelectrodes may be established on the other principal plane of thethermistors. In another manner, two or more electrodes can be providedto both of the principal planes of the PTC thermistors.

The results of the present Example can be summarized as follows.

(1) The power output of the PTC heater according to the present Examplecan be varied by using a single heating element. Accordingly, a far morecompact PTC heater than is conventional can be provided at a reducedcost.

(2) The power output of the PTC heater according to the present Examplecan be divided arbitrarily into steps by increasing the number ofelectrodes. Accordingly, the PTC heaters serve as more comfortablegeneral use heaters.

(3) The PTC heater according to the present Example comprises openspaces on the side provided thereon with a plurality of electrodes. Itcan be seen that the heat conductivity of such a side is impaired ascompared with that of a side provided thereon with a single electrode.Accordingly, by applying the PTC heater of the present Example for adevice which emits heat only from one side of the panel, such as a panelheater for floor heating, heat loss can be reduced and anenergy-efficient device can be realized, provided that the side providedwith a plurality of electrodes is used as the back side of the heater.

EXAMPLE 7

FIG. 23 is a perspective view of a PTC thermistor heater device of thepresent invention according to Example 7, which comprises an upperelectrode sheet 54 and a lower electrode sheet 55. Brass sheets each 800mm×25 mm×0.3 mm in size were used as the electrode sheets in the presentExample. The device comprises equally spaced PTC ceramic elements 56being incorporated between the upper and the lower electrode sheets 54and 55. Barium titanate (BaTiO₃) ceramics, for example, may be used asthe PTC ceramic elements 56. A mica sheet was used as the insulatingmaterial 57 because of its excellent heat resistance and insulationproperties. The insulating material 57 provided was at the samethickness as the PTC ceramic element 56. More specifically, a mica sheetmachined to a thickness of 2.5 mm was adhered to the electrode sheets 54and 55 with an adhesive.

Material other than a mica sheet may be used as the insulating material57 so long as it has excellent insulating characteristics and heatresistance. For example, ceramic materials can be favorably used fromthe viewpoint of realizing a uniform temperature over the entirematerial. Furthermore, from the viewpoint of improving thermalefficiency and safety, the PTC elements and the insulating sheet may besubjected to potting to charge therebetween a resin based material andthe like.

A large-area panel can be realized by arranging several PTC thermistorheater devices as above, parallel with each other and equally spaced.

As mentioned in the foregoing, the present Example provides a heatercomprising an insulating material incorporated into the open spacesbetween the electrode sheets, thereby improving in strength and safety.

The present invention can be applied to home-use floor heaters and toheaters for warming foodstuffs.

We claim:
 1. A PTC heater device comprising a PTC element includingBaTiO₃ as a principal component thereof and Nb₂ O₅ as an additivethereof, the amount of Nb in said PTC element is limited to less than0.03% by weight, thereby suppressing a rush current.
 2. A PTC heaterdevice comprising a PTC element having a surface portion and an innerportion, with a ratio of resistivity of the surface portion toresistivity of the inner portion of 1.1-0.8 along the direction ofthickness, thereby decreasing a rush current.
 3. A PTC heating device asclaimed in claim 1 or claim 2, wherein the device yields a product ofroom temperature resistance and output power at 25° C. of 2.0×10⁴ (Ω·W)or higher, thereby reducing a rush current.
 4. A PTC heating deviceincluding a PTC element having an ∝ value in the range of from 10 to20%/°C., the device yielding a product of room temperature resistanceand output power at 25° C. of 2.0×10⁴ (Ω·W) or higher, thereby reducinga rush current.
 5. A method of reducing rush current in a PTC element ofa PTC heater device comprising utilizing BaTiO₃ as a principal componentof said PTC element and Nb₂ O₅ as an additive thereof, limiting saidBaTiO₃ to contain less than 0.03% by weight of Nb, and thereby providinga PTC element having reduced rush current.
 6. A PTC element havingreduced rush current made according to the method of claim 5.